This invention relates generally to television apparatus, and more particularly to a method and apparatus for digitally encoding an NTSC color television signal.
Where a transmission channel has a limited data rate or a digital store has a limited capacity, it sometimes becomes necessary to reduce the data rate of pulse-code-modulated (PCM) color television signals. One direct way of doing this is to lower the encoding frequency, f.sub.s. However, the Nyquist sampling limit is reached when f.sub.s = 2f.sub.v, where f.sub.v is the highest video frequency being encoded. Reducing f.sub.s further causes beating distortions due to the lower sidebands of f.sub.s overlapping the baseband video frequencies. For NTSC television, f.sub.v = 4.2 MHz and the Nyquist sampling limit is 8.4 MHz.
With a monochromevideo signal, if f.sub.s is chosen to be an odd integral multiple of one-half the line scan frequency, f.sub.h, it is possible to remove most of the aliasing distortion by comb filtering after the codec; a system for doing this is described in an article by L. S. Golding entitled, "A Digital Communications System for Satellite Links", Proceedings of Second International Conference on Digital Satellite Communications, Paris, November 1972. This is true because the alias components are largely confined to bursts of energy centered on (n + 1/2) f.sub.h, where n is an integer, and the luminance component (Y) of the video signal is confined to bursts centered on n f.sub.h. Only those frequencies lying between f.sub.s - f.sub.v and f.sub.v need be comb filtered.
The unique characteristics of PAL color television signals permit their being encoded at sub-Nyquist rates and comb filtered to remove aliasing, in the manner described by V. G. Devereux and G. J. Philips in an article entitled, "Bit-Rate Reduction of Digital Video Signals Using Differential PCM Techniques", pp. 83-89, IEEE Conference Publication No. 119. This is possible due to the chrominance (C) signal energy being centered on bursts of frequencies at (n+1/4)f.sub.h. When the PAL signal is encoded at a sub-Nyquist frequency equal to an odd integral multiple of 1/2f.sub.h, the Y alias components are restricted to frequencies centered on (n+1/2)f.sub.h, which can then be placed in the nulls of the comb filter for removal.
NTSC television signals could also be digitally encoded at sub-Nyquist rates if means were available to place the alias components into those parts of the spectrum not occupied by Y or C components of the video signal. It would then be possible, by suitable comb filtering, to remove most of the alias signals from the baseband video. It is a primary object of the present invention to provide a method and apparatus for satisfying this need, thereby to reduce the data rate of PCM-encoded NTSC television signals.